The Wisconsin DOT states:
347.38 Horns and warning devices.
347.38(1) <http://docs.legis.wisconsin.gov/document/statutes/347.38(1)> (1)
No person shall operate a motor vehicle upon a highway unless such motor
vehicle is equipped with a horn in good working order and capable of
emitting sound audible under normal conditions from a distance of not less
than 200 feet
I once had a buddy out there in Milwaukee that considered himself complaint
even though his horn was dead, so long as he had his wife with him!
They are no longer together....
Keith